Promax 85mm mini-V, need replacement

I have a Schwinn Super Sport flat-bar road bike.

After 1600 miles, the brakes feel mushy. Probably housing/cables need to be replaced.

It comes with Promax 85mm mini-V (V brake). So, I had this idea to replace the brakes with Avid SD7 (100mm, $19 per). One problem..they don't fit!! Or, can I replace the stud that screws into the boss?

Huh...that's a bit strange. I can say, though, that I've run several sets of brakes that don't fit flush onto the posts with no issues. But, that raised area at the spring pin base is definitely a kicker. I'm fresh out of brilliant ideas!